You need to try them out for yourself rather than going solely off of other people's impressions, because your ears are not our ears, so our impressions will differ from yours.

I've tried them twice, and they sound very weird, but I don't think it's as bad as some people make it out to be. It's always going to sound "thin", like vocals don't have much weight or body to them. If the vocals require the lower midrange (e.g. many male vocals), it can start to sound hollow, like sound is coming from a very echo-ey chamber. The way I would put it, for male vocals, is that it makes every vocalist go up one pitch, like bass becomes baritone, baritone becomes tenor, and tenor becomes basically falsetto. It immediately sounds weird for male vocalists. I think female vocalists are less affected, especially if they already have a higher-pitched or thinner voice.

The treble can be a bit grainy and uneven, but not harsh. It's not a fatiguing sound. The bass sounded pretty good to my ears, with decent quantity and quality. And the soundstage is one of the widest I've heard, not just amongst closed-backs, but in general. But the midrange is one of the strangest I've heard, and it's a dealbreaker for a lot of people.

Definitely one to try before you buy. I'm guessing a lot of people bought it blind because it was the top model in Sennheiser's lineup and were immediately put off by the bizarre midrange and wanted to sell it. The 820 is the headphone I most want to listen to more, to fully understand its weirdness, but I don't feel like spending $1800 on weird right now.